Aug 4, 2013

Photosynthesis – takes
place in the chloroplast
There two main stages :

i) light reaction and

ii) dark reaction

Light reaction occurs only in the presence of light. Dark reaction occurs
during day and night.Light reaction - occurs in grana.
Chlorophyll captures light energy - excites the electrons to higher energy
levels.
Electrons then leave the chlorophyll.
Light energy is also used to split water molecule into hydrogen ions and
hydroxyl ions.

This is known as photolysis of water.
Hydrogen ions combine with electrons released by chlorophyll to form hydrogen
atoms.
The energy from excited electrons is used to form adenosine triphosphate (ATP).
At the same time, hydroxyl ion loses an electron to form hydroxyl group.
This electron is then received by chlorophyll.
The hydroxyl groups then combine to form water and gaseous oxygen.
Oxygen is released into the atmosphere and used for cellular respiration.

The ATP
molecules provide energy while the hydrogen atoms provide reducing power for
the dark reaction.Dark reaction - also known as Calvin cycle.
Occurs in stroma.

Hydrogen atoms are used to fix carbon dioxide into a series of
reactions catalysed by photosynthetic enzymes.
Carbon dioxide is reduced into glucose.
Glucose monomers undergo condensation to form starch which will stored
temporarily as starch grains in the chloroplast.

ConclusionLight reaction - occurs in the grana (that contained chlorophyll) - takes place in
the presence of sunlight and chlorophyll - chlorophyll absorbs light; then it
becomes activated and this energy is used to :i) produce energy in the form
of – ATP (used for dark reaction)ii) split up water molecules (photolysis)
into hydroxyl ions(OH-) and hydrogen ions (H+) - oxygen is released; but
hydrogen enters dark reaction.Dark
reaction (Light independent reaction) - takes place in the stroma -ATP combined
with hydrogen atoms (from the light reaction) are used to reduce carbon dioxide
to form glucose.Glucose produced
–i) converted to starch (stored),ii) transformed - sucrose ; transported
to other partsiii) synthesis of celluloseiv) converted to amino acids
and fatty acids